A tempered glass is used as a cover glass of a portable telephone, a digital camera, a PDA and so on, or as a substrate glass of a touch panel and so on. In recent years, these devices have become widely used, and accordingly, there is a strong demand for producing these in a large amount at low cost.
A glass substrate used in these applications is required to be high in the mechanical strength. In this connection, so-called chemically tempered glass that is tempered through an ion exchange process and the like has been proposed (for instance, see JP-A-2006-83045 and JP-A-2007-31211).
However, in conventional tempered glasses, it takes a long time to apply an ion exchange process to be high in cost. Furthermore, since devices such as portable telephones are required to be light in weight and small in size, glass substrates used in the devices are required to be thin in the thickness and low in the density. However, since conventional tempered glasses are inferior in the devitrification, only a process such as a roll forming process can be used, whereby it is difficult to form a thin sheet. Accordingly, a process such as polishing has to be applied after the formation to make thinner.